minor symfony#6300 Document constraint validator alias optional (Triiistan)

This PR was merged into the 2.3 branch.

Discussion
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Document constraint validator alias optional

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no (symfony/symfony#17074)
| Applies to    | 2.3
| Fixed tickets |

Same as symfony#6055 but on the 2.3 branch as the relevant Symfony PR symfony/symfony#17074 was merged on 2.3.

Commits
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873ede0 Document constraint validator alias optional

Author: weaverryan

cookbook/validation/custom_constraint.rst

@@ -161,7 +161,7 @@ Constraint Validators with Dependencies
 If your constraint validator has dependencies, such as a database connection,
 it will need to be configured as a service in the Dependency Injection
 Container. This service must include the ``validator.constraint_validator``
-tag and an ``alias`` attribute:
+tag and may include an ``alias`` attribute:
 
 .. configuration-block::
 
@@ -189,21 +189,14 @@ tag and an ``alias`` attribute:
             ->register('validator.unique.your_validator_name', 'Fully\Qualified\Validator\Class\Name')
             ->addTag('validator.constraint_validator', array('alias' => 'alias_name'));
 
-Your constraint class should now use this alias to reference the appropriate
-validator::
+As mentioned above, Symfony will automatically look for a class named after
+the constraint, with ``Validator`` appended. You can override this in your constraint class::
 
     public function validatedBy()
     {
-        return 'alias_name';
+        return 'Fully\Qualified\ConstraintValidator\Class\Name'; // or 'alias_name' if provided
     }
 
-As mentioned above, Symfony will automatically look for a class named after
-the constraint, with ``Validator`` appended. If your constraint validator
-is defined as a service, it's important that you override the
-``validatedBy()`` method to return the alias used when defining your service,
-otherwise Symfony won't use the constraint validator service, and will
-instantiate the class instead, without any dependencies injected.
-
 Class Constraint Validator
 ~~~~~~~~~~~~~~~~~~~~~~~~~~